Telomere shortening, a natural consequence of aging, and telomerase dysfunction have recently been linked with certain diseases associated with defects in highly proliferative tissue, including the epidermis. Our long-term goal is to understand the contribution of telomerase and telomere length in regulating the function and aging of human skin. Dyskeratosis congenita (DC) is an inherited "premature aging" syndrome characterized by epidermal abnormalities (e.g. leukoplakia, skin hypopigmentation, alopecia) and organ dysfunction (e.g. bone marrow failure, pulmonary fibrosis). DC is caused by mutations in genes that affect telomerase activity, resulting in shortening of telomeres in somatic cells. We recently identified an autosomal dominant form of this disease (AD DC) that is due to a deletion in hTERC, the RNA component of telomerase. AD DC provides a unique human model system to study the regulatory role of telomerase and telomere shortening in the biology of aging. We propose that telomerase and telomere length are important regulators of normal keratinocyte function, differentiation, and response to stress. Furthermore, we hypothesize that skin aging is associated with telomere length shortening and dysregulation of telomerase component expression. Using skin keratinocytes and fibroblasts isolated from AD DC subjects, normal controls and experimentally manipulated cells, we will further define the role of telomerase and telomere length in regulating DNA damage and oxidative stress response pathways. We will determine whether introduction of telomerase components, hTERC and/or hTERT, into AD DC cells restores replicative potential, cellular function, and normal responses to stress and DNA damage. We will also explore the possibility that hTERT has functions in keratinocytes that go beyond telomere maintenance and perform studies to determine whether the unusually long telomeres associated co-expression of hTERC and hTERT have any effects on cell phenotype. Overall, the experiments proposed in this application will further our understanding of the role of telomerase and telomeres in cellular defects associated with aging, and provide critical mechanistic insights into the processes that govern normal skin function and aging. Importantly, these findings may also promote the development of novel strategies aimed at reducing the effects of aging on human skin and other tissues, and in ameliorating the toxicity of cancer therapies in the elderly.